1. Field of the Invention
The present invention relates in general to a via for providing a signal path between conductors formed on separate layers of a printed circuit board, and in particular to a via that acts as a tuned filter to optimize characteristics of its frequency response.
2. Description of Related Art
FIGS. 1 and 2 are plan and sectional elevation views of a portion of a prior art printed circuit board (PCB) employing a conductive via 12 to link a microstrip conductor 14 formed on an upper surface 16 of the PCB to a microstrip conductor 18 formed on a lower surface 20 of the PCB. Via 12 includes an upper cap (annular ring 22) contacting conductor 14, a lower cap (annular ring 24) contacting conductor 18 and a vertical conductor 26 extending between upper and lower annular rings 22 and 24. PCB 10 also includes embedded power and ground planes 28 formed on PCB substrate layers below upper surface 16 and above lower surface 20, and may also include additional embedded power, ground or signal planes 30. Via conductor 26 passes though holes in power signal planes 28 and 30 sufficiently large to prevent conductor 26 from contacting planes 28 and 30.
FIG. 3 is an impedance model of the path a signal follows through conductor 14, via 12 and conductor 18. Microstrip conductors 14 and 18, modeled by their characteristic impedances Z1 and Z2 respectively, are often sized and spaced with respect nearby power or ground planes 28 so that they have a standard characteristic impedance such as 50 Ohms. Upper and lower via annular rings 22 and 24 add shunt capacitances C1 and C2 to the signal path provided by vertical conductor 26. An inductor L1 models the vertical conductor 26. The model of FIG. 3 could also include some shunt resistance to account for leakage though the insulating substrate surrounding via 12, but at higher frequencies capacitances C1 and C2 and inductance L1 are the predominant influences on the via""s frequency response.
Via 12, which behaves like a three-pole filter or passive network, can severely attenuate and distort a high frequency signal traveling between conductors 14 and 18. The series inductance L1 provided by vertical conductor 26 depends primarily on its vertical dimension. Since vertical conductor 26 must extend through PCB 10, its length is fixed by the thickness of PCB 10, and there is generally little leeway in adjusting the value of L1 . Thus the conventional approach to reducing signal distortion and attenuation caused by via 12 in high frequency applications has been to minimize the via""s shunt capacitance. The shunt capacitance C1 and C2 can be reduced by reducing the horizontal dimension of annular rings 22 and 24 and by maximizing the distance between annular rings 22 and 24 and nearby power and ground planes 28. However there are practical limits to the amount by which capacitances C1 and C2 can be reduced. Therefore appreciable via capacitance and inductance will always be present and will always cause some level of signal distortion and attenuation, particularly in high frequency signals.
FIG. 4 includes a plot A of the frequency response of a typical via that has been designed to provide minimal shunt capacitance and series inductance. The bandwidth of a filter is normally defined as the lowest frequency at which it attenuation reaches xe2x88x923db. Plot A of FIG. 4 shows that the bandwidth of via 22 is approximately 3.2 GHz. Thus a circuit board designer would normally want to avoid using such a circuit board via to conduct a signal of frequency higher than about 3 GHz.
The conventional approach to the use of vias in high frequency applications is therefore quite often to avoid them entirely. However a restriction against using vias can make it difficult to route large numbers of high frequency signals on a circuit board. In some high frequency applications short xe2x80x9cblindxe2x80x9d vias which do not extend completely through a PCB are used to link embedded stripline conductors formed on PCB layers that are vertically close to one another. Since blind vias are short, they have relatively little series inductions, and therefore usually have larger bandwidths than through vias extending completely through a PCB. However blind vias are more expensive than through vias, and still do not have sufficient bandwidth to handle very high frequency signals.
Therefore what is needed is a way to substantially increase the bandwidth of PCB vias so that they can conduct very high frequency signals without unduly attenuating or distorting them.
A printed circuit board (PCB) via provides a vertical signal path between microstrip or stripline conductors formed on separate horizontal layers of a PCB. The via adds shunt capacitance and series inductance to the signal path that are functions of shape and size of the via and of spacing between the via and nearby power and ground planes implemented in the PCB.
In accordance with one aspect of the invention, the capacitances of the via are adjusted with respect to one another and to the via inductance to values above their minimum practically attainable values for which frequency response characteristics of the via such as bandwidth are optimized.
In accordance with another aspect of the invention, in particular embodiments thereof, the via capacitances are adjusted so that the via behaves as a multi-pole Chebyshev or Butterworth filter.
In accordance with a further aspect of the invention, the via includes a capacitive element embedded within the PCB in contact with the signal path provided by the via. The element""s shunt capacitance and the magnitudes of capacitances of other portions of the via are adjusted relative to via""s inherent series inductance and to the impedance of the stripline or microstrip conductors to tune the via for optimal frequency response characteristics.
It is accordingly an object of the invention to provide a PCB via conducting high frequency signals without unduly attenuating them.
The claims portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.